Periodic Reporting for period 2 - AxionDM (Searching for axion and axion-like-particle dark matter in the laboratory and with high-energy astrophysical observations)
Reporting period: 2023-01-01 to 2024-05-31
The nature of the dark matter, which makes up more than 80% of the Universe’s matter content, remains unknown. It could be made up of so-far undetected fundamental particles. One such candidate is the axion or an axion-like particle, which is predicted by numerous theories extending the Standard model of particle physics. These particles could be detected through their oscillations into photons in the presence of magnetic fields. With my research, I conduct complementary laboratory and astrophysical searches for dark-matter axions and ALPs that cover a broad range of possible axion masses, which is completely unknown. The discovery of the axion or an ALP would mark the detection of the first elementary particle since the discovery of the Higgs Boson and would be the first direct evidence of physics beyond the Standard model.
Regarding the astrophysical searches, my team and I have performed searches for signatures of photon-axion oscillations using observations of certain types of galaxies, called blazars. Such blazars host supermassive black holes in their center and produce energetic outflows of particles. These so-called jets are also the source of photons with wavelengths covering the entire electromagnetic spectrum from radio waves up to gamma-ray energies. Special telescopes are able to measure spectra of these sources at gamma-ray energies such as the Large Area Telescope (LAT) on board the Fermi satellite or the ground-based High Energetic Stereoscopic System (H.E.S.S.). We have used gamma-ray spectra of several blazars measured with the LAT and H.E.S.S. to look for energy-dependent oscillations caused by photon-axion interactions. We have not found any signs of such oscillations. As a result, we have placed novel limits on the strength of the photon-axion interaction. For certain axion masses, the bounds are amongst the strongest to date. The code that calculates the probability for axion-photon conversion in different astrophysical environments has been released to the public under the name gammaALPs. Using LAT and H.E.S.S. observations, we have also been able to place a novel set of constraints on the strength of the magnetic field in between galaxies, which is an important ingredient for the searches for photon-axion interactions.
Concerning laboratory searches, my team and I have successfully joined the ALPS II collaboration and significantly contributed to the development of a sensitive detector that is able to detect individual photons. The ALPS II experiment is a so-called light shining through the wall experiment in which a powerful laser is shone on an opaque wall. By placing the laser beam string of magnets, some of the laser photons should convert into axions, traverse the wall and reconvert into photons in a second magnet string behind it. The ALPS II experiment has unprecedented sensitivity for the production and subsequent detection of axions thanks to the two 120m long strings of magnets with a field strength of over 5 Tesla. A first data taking campaign has started in 2023.
The main contributions of my team concern one of the foreseen detectors for the reconverted photons. This detector is essentially a superconducting thermometer operated at a temperature close to absolute zero. Our preliminary results suggest that this detector has an efficiency of 90% to detect an incident photon. Furthermore, we have developed an analysis pipeline to distinguish photons from reconverted axions from background signals using machine learning and neural networks. Our results indicate that we can efficiently suppress backgrounds below the required level.
Concerning laboratory searches, my team and I have successfully joined the ALPS II collaboration and significantly contributed to the development of a sensitive detector that is able to detect individual photons. The ALPS II experiment is a so-called light shining through the wall experiment in which a powerful laser is shone on an opaque wall. By placing the laser beam string of magnets, some of the laser photons should convert into axions, traverse the wall and reconvert into photons in a second magnet string behind it. The ALPS II experiment has unprecedented sensitivity for the production and subsequent detection of axions thanks to the two 120m long strings of magnets with a field strength of over 5 Tesla. A first data taking campaign has started in 2023.
The main contributions of my team concern one of the foreseen detectors for the reconverted photons. This detector is essentially a superconducting thermometer operated at a temperature close to absolute zero. Our preliminary results suggest that this detector has an efficiency of 90% to detect an incident photon. Furthermore, we have developed an analysis pipeline to distinguish photons from reconverted axions from background signals using machine learning and neural networks. Our results indicate that we can efficiently suppress backgrounds below the required level.
Our results on axion searches using blazars signify an important step beyond the state of the art. On the one hand, the derived bounds are strongest to date derived from gamma-ray observations. On the other hand, our magnetic field model self-consistently describes the broad band emission of the blazar. For the first time, we also incorporated the uncertainty of the intervening magnetic fields in our statistical analysis, which renders our results more robust. Furthermore, our constraints on the intergalactic magnetic field improve previous bounds by a factor of two and combine data from different instruments self-consistently.
For the remaining time, my team and I will focus on the search for heavier axions that could decay into photons. Such a decay should be visible in background radiation fields that penetrate the universe. These background fields are not precisely known and we will model the astrophysical mechanisms in addition to the axion decay self-consistently and confront our predictions with data. Furthermore, we will conduct novel searches for axions produced in supernova explosions. In such explosions that occur at the end of the lifetime of massive stars, axions could be copiously produced and subsequently convert to gamma rays in the magnetic field of our Milky Way. As a result, we expect a short gamma-ray burst that we will look for in Fermi LAT data. We have already started to collect a sample of archival supernova explosions detected with optical telescopes that are well suited for such searches.
Regarding laboratory searches, my team will continue to characterize our single photon detector and improve the suppression of backgrounds. Our preliminary results on the efficiency together with the achieved background suppression mark an important step beyond the state of the art for such detectors. We will further investigate the response of the detector to photons at different wavelengths in order to calibrate its energy resolution. We will also work on the improvement of machine learning algorithms for signal and background discrimination. In particular, convolutional neural networks for time series classification offer an exciting opportunity in this regard. Lastly, we will develop an optical filter bench that can be operated within the cryostat that also houses the detector. Such a filter bench can in principle reject photons at the wrong wavelength but we will have to ensure a high transmission at the correct wavelength.
For the remaining time, my team and I will focus on the search for heavier axions that could decay into photons. Such a decay should be visible in background radiation fields that penetrate the universe. These background fields are not precisely known and we will model the astrophysical mechanisms in addition to the axion decay self-consistently and confront our predictions with data. Furthermore, we will conduct novel searches for axions produced in supernova explosions. In such explosions that occur at the end of the lifetime of massive stars, axions could be copiously produced and subsequently convert to gamma rays in the magnetic field of our Milky Way. As a result, we expect a short gamma-ray burst that we will look for in Fermi LAT data. We have already started to collect a sample of archival supernova explosions detected with optical telescopes that are well suited for such searches.
Regarding laboratory searches, my team will continue to characterize our single photon detector and improve the suppression of backgrounds. Our preliminary results on the efficiency together with the achieved background suppression mark an important step beyond the state of the art for such detectors. We will further investigate the response of the detector to photons at different wavelengths in order to calibrate its energy resolution. We will also work on the improvement of machine learning algorithms for signal and background discrimination. In particular, convolutional neural networks for time series classification offer an exciting opportunity in this regard. Lastly, we will develop an optical filter bench that can be operated within the cryostat that also houses the detector. Such a filter bench can in principle reject photons at the wrong wavelength but we will have to ensure a high transmission at the correct wavelength.